362 IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 52, NO. 3, MARCH 2004 CI/FSK: Bandwidth-Efficient Multicarrier FSK for High Performance, High Throughput, and Enhanced Applicability Balasubramaniam Natarajan, Member, IEEE, Carl R. Nassar, Senior Member, IEEE, and Steve Shattil Abstract—In traditional binary frequency-shift keying (BFSK), two distinct carrier frequencies are used to represent binary data. In this letter, we propose transmitting multiple orthogonal, in-phase subcarriers around two distinct carrier frequencies to represent binary information. The envelope of the resulting FSK signal is a carrier interferometry pattern, and hence, the name carrier interferometry (CI/) FSK. We demonstrate that this tech- nique is spectrally efficient when compared to traditional BFSK. Performance and data-rate benefits are also demonstrated with the use of a novel coherent reception technique. Moreover, while it is difficult to employ traditional BFSK in multipath channels, we show how the new CI/FSK scheme exploits frequency-diversity benefits when used in such channels. Index Terms—Carrier interferometry (CI), multicarrier (MC) modulation, frequency-shift keying (FSK). I. INTRODUCTION F REQUENCY-SHIFT keying (FSK) is a modulation tech- nique of great practical importance. It is commonly used whenever the hardware simplicity of the receiver is of utmost importance [1]. Recently, FSK has gained popularity with its adoption in the Bluetooth standard [2]–[4]. The selection of FSK for personal area networks (such as Bluetooth) was motivated by a number of considerations. Noncoherent FSK receivers can be designed with ease and can be implemented in a cost-effec- tive manner. Since most FSK modulation techniques result in a constant envelope, information is carried by the zero cross- ings of the signal alone. Hence, FSK is robust in systems that have nonlinearities due to, e.g., radio frequency (RF) amplifier effects [5]. However, FSK also has significant disadvantages. Since the binary FSK (BFSK) constellation is orthogonal rather than antipodal, it suffers from a 3-dB penalty in signal-to-noise ratio (SNR) for a given bit-error rate (BER) [when compared with coherent binary phase-shift keying (BPSK)]. Additionally, the spectral efficiency of FSK is often lower than passband pulse-amplitude modulation (PAM) and phase-shift keying (PSK). Next, since the basic FSK signal is not a linear function of the data, existing linear-equalization techniques cannot be Paper approved by C. Tellambura, the Editor for Modulation and Signal De- sign of the IEEE Communications Society. Manuscript received January 28, 2002; revised August 8, 2003. This paper was presented in part at the IEEE In- ternational Conference onCommunications, New York, NY, May 2002. B. Natarajan is with the Department of Electrical and Computer Engi- neering, Kansas State University, Manhattan, KS 66506-5204 USA (e-mail: bala@eece.ksu.edu). C. R. Nassar is with the Department of Electrical and Computer Engi- neering, Colorado State University, Ft. Collins, CO 80523-1373 USA (e-mail: carln@engr.colostate.edu). S. Shattil is with Idris Communications, Boulder, CO 80301 USA (e-mail: steve@idriscomm.com). Digital Object Identifier 10.1109/TCOMM.2004.823570 used. Hence, compensation for channel distortion like selective fading is more difficult in FSK. As a result, multipath channels introduce an error floor (even in the absence of noise) [6]. This major drawback hinders the widespread use of FSK in wireless systems [1]. There has been considerable interest in designing FSK detec- tors for multipath channels (see [7]–[12]). In [7], an asymmetric raised-cosine pulse shape is applied, as is a limiter discrimi- nator receiver, improving the performance of FSK in multipath channels with small delay spreads. However, when the delay spread increases above the symbol duration, a significant error floor appears. Additionally, the system in [7] introduces a nom- inal degradation in spectral efficiency relative to Gaussian FSK (GFSK). In [8], performance gains in Rayleigh fading channels with small interpath delays are achieved via a quadratic decor- rellation receiver. In [9], a generalization of frequency modula- tion (FM) noise-click theory is used to design an FSK receiver for detection after multipath spreading. In both [8] and [9], there is significant degradation in performance when the delay spread approaches symbol duration. A novel noncoherent equalizer for FSK is introduced in [10]. Here, intersymbol interference (ISI) is combated using an approach similar to the phase-indepen- dent decorrellator multiuser detection employed in multiple-ac- cess systems (where interference from other users is tuned out). However, just as in multiuser detection, complexity is a limiting factor in this approach. Recently, in [11] and [12], the idea of exploiting path diversity in FSK systems is explored, and it is shown to yield nominal performance gains. The impact of in- terpath interference, along with hardware limitations (restricting the number of RAKE fingers that can be implemented), reduces this system’s ability to benefit from the available diversity. In this letter, we propose an enhancement to BFSK that: 1) provides improved spectral efficiency; 2) supports enhanced BER performance; and 3) ensures the successful transmission of FSK over frequency-selective channels, by supporting frequency diversity benefits. Specifically, in traditional BFSK, carrier frequency represents binary “1,” and carrier frequency indicates binary “0.” In the new system, we use orthogonal in-phase subcarriers around to represent one, and a second set of subcarriers centered about to represent zero. The total transmitted signal has an average frequency equal to either or , while the envelope corresponds to a car- rier interferometry (CI) pattern. Hence, this novel FSK system is referred to as carrier interferometry/FSK (CI/FSK), and can be understood as a synergistic combination of orthogonal fre- quency-division multiplexing (OFDM) and FSK. We first demonstrate that CI/FSK has a spectral efficiency comparable to that of FM with a modulating waveform, 0090-6778/04$20.00 © 2004 IEEE